Pockels cells

ABSTRACT

An electro-optical cell, comprising an insulating case, having two windows arranged opposite to each other; a crystal in said insulating case; two insulating countercheeks, supported on the extreme faces of said crystal, by means of metallic electrodes, said countercheeks covering said windows in said insulating case, said electrodes and countercheeks having openings to allow the passage of light traversing the crystal;, and further characterized in that a sleeve, consisting of an insulating substance, is arranged around a lateral surface of said crystal and is in contact with said lateral surface and the interior surface of said case opposite said lateral surface, so as to insulate electrically said electrodes from each other is disclosed.

United States Patent Boutineau Jean-Louis Boutineau, Antony, France Compagnie Generale D'Electricite, Paris, France [22] Filed: Mar. 26, 1970 [21] Appl.No.: 22,831

11 1 3,659,917 1 May2, 1972 FOREIGN PATENTS OR APPLICATTONS 1,433,422 1966 France ..350/l OTHER PUBLICATIONS lsom et Electro Optic Light Modulators, Bulletin 1201, 3/65 R. Williams, Physical Review, Vol. 126, No. 2, 4/15/62 pp. 442- 446 Primary Examiner-R. L. Wibert Assistant Examiner-Jeff Rothenberg AItorney-Sughrue, Rothwell, Mion, Zinn & Macpeak 57] ABSTRACT An electro-optical cell, comprising an insulating case, having two windows arranged opposite to each other; a crystal in said insulating case; two insulating countercheeks, supported on the extreme faces of said crystal, by means of metallic electrodes, said countercheeks covering said windows in said insulating case, said electrodes and countercheeks having openings to allow the passage of light traversing the crystal;, and further characterized in that a sleeve, consisting of an insulating substance, is arranged around a lateral surface of said crystal and is in contact with said lateral surface and the interior surface of said case opposite said lateral surface, so as to insulate electrically said electrodes from each other is disclosed.

3 Claims, 6 Drawing Figures [30] Foreign Application Priority Data Mar. 28, 1969 France ..6909472 [52] U.S. C1. ..350/l60, 350/150, 350/319 [51] Int. Cl. ..G02I 1/34 [58] Field oiSeareh ..350/150, 160, 318, 319

[56] References Cited UNITED STATES PATENTS 3,454,325 7/1969 Ohm ..350/l 3,357,771 12/1967 Buhrer.... ..350/ 3,431,418 3/1969 Stone ..350/l50 3| J i I,

Patented May 2, 1972 3,659,917

3 Sheets-Sheet 1 FIG I PRIOR ART FIG. I

Patented May 2, 1972 3 Sheets-Sheet 2 Patented May 2, 1972 3,659,917

3 Sheets-Sheet 5 i FIG. (5

POCKELS CELLS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to electro-optical cells, such as Pockels cells, and is intended to provide a new structure permitting the obtainment of a maximum transmission of a light beam with the application of very high electrical voltages to the cell.

2. Description of the Prior Art It is known that certain crystals of the potassium diacid phosphate type (KDP) or deuterium-treated KB? or ammonium di-acid phosphate (ADP) present a strong Pockels effect when an electrical field is applied in the direction of the crystallographic axis. These crystals have the disadvantage of being very hygroscopic; they can be protected against humidity by glueing, on each of the faces of the crystal, a protection window which may be made of glass or quartz, for example, using a silicon grease having a high viscosity or a glue. An electrical voltage may be applied to the crystal by means of two electrodes placed on the protection windows; these electrodes may consist of a metal film, such as aluminum, copper, or indium, or they may consist of metal oxides, such as cadmium oxide, indium oxide, or tin oxide, or they may be of metallic plates in various shapes deposited in a vacuum or glued on; for example, it is known that electrodes come in the form of a grid or a ring.

The Pockels effect increases as the electrical field becomes more intensive and the protecting windows must have a dielectric constant as high as possible. This requirement limits the choice of materials which can be used.

The application of an electrical field may also be accomplished with the help of two electrodes deposited under a vacuum or glued, using a cement, on the faces of the crystal. The extreme faces of the crystal are then protected by two windows which may be made of glass or quartz. In these known structures of electro-optical cells, the crystal equipped with its protection windows, is held mechanically, for example, in a glass or synthetic resin sheath.

It is impossible to achieve perfect contact between the protection sheath and the lateral surface of the crystal and when a high voltage is applied to the latter, a breakdown in the air gap between this protection sheath and the crystal occurs. The voltage applied to the known cells may, in practice, not exceed 9 to K volts.

In order to achieve better insulation, a liquid, such as a silicon oil, is interposed all around the crystal, with the liquid being held in place by the protection windows. In this case, it is necessary to select the liquid wisely. Such a liquid must be compatible not only with the nature of the crystal but also with the material of the window. This liquid furthermore must possess good optical transmission characteristics. Finally, this liquid may gradually evaporate and thus render the cell useless.

Protecting windows must be polished perfectly and their faces must be parallel exactly to those of the crystals. Where this condition is not achieved, the transmission of the cell is diminished considerably, which influences the quality of the optical beam traversing it.

This invention is intended to eliminate these disadvantages and to provide a Pockels cell which does not have protection windows, in other words, with maximum transmission, but having means for the protection of the faces of the crystal and means creating electrical insulation, permitting the application of very high voltages.

SUMMARY OF THE INVENTION This invention concerns an electrocptical cell comprising an insulating case having two openings arranged opposite to each other. The crystal has two flat faces perpendicular to the crystallographic axis and also has a lateral surface. Two insulating countercheeks are supported on the faces by means of metallic electrodes. The countercheeks cooperate with the insulating case and cover the openings. The electrodes and countercheeks have openings to allow the passage of light traversing the crystal. A sleeve consisting of an insulating substance is arranged around the lateral surface and is in contact with the lateral surface and the inner surface of the case 0pposite the lateral surface, thereby electrically insulating the electrodes from each other.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in detail with reference to the attached drawings given here by way of illustration and without limitation.

FIG. 1 shows a schematic cross-section of a known Pockels cell.

FIG. 2 shows a crosssection view of a Pockels cell according to this invention.

FIG. 3 shows a cross-section view cell according to the invention.

FIG. 4 represents a cross-section view of another variation of a Pockels cell according to the invention.

FIGS. 5 and 6 represent cross-section views of variations of Pockels cells according to this invention.

DETAILED DESCRIPTION OF THE INVENTION In the prior art system of FIG. 1, a crystal 1, for example, a crystal of KDP, whose flat faces 2 and 3 are perpendicular to the crystallographic axis and are equipped with electrodes 4 and 5 deposited in a vacuum, is protected by two windows 6 and 7, which may be made ofglass or quartz, glued upon faces 2 and 3. The lateral faces of the crystal are protected by the cylindrical body 8 made of glass or synthetic resin, for example.

The entire assembly is arranged inside a sleeve 19, usually made of insulating material, such as, for example, a synthetic resin or a similar substance. The crystal, with its protection windows, may be held mechanically in the body by countercheeks, not illustrated here. It is impossible to achieve perfect mechanical contact between crystal 1 and body 8 and between body 8 and sleeve 19. When a high voltage is applied to the crystal by means of connections 11 and 12, a breakdown in the air gap between crystal 1 and body 8 or possibly between body 8 and sleeve 19 occurs.

According to another known structure, drawings,

of a variation of a Pockels not shown in the the electrodes are deposited or glued on the internal faces of the protecting windows, these windows being then glued on the crystal. The mounting which holds the crystal is identical to the mounting described earlier and thus involves the disadvantage of breakdowns.

As shown in FIG. 2, illustrating a cell according to the invention, electrical insulation can be improved by introducing an insulating liquid into the cell and by using only one sleeve. The cell in FIG. 2 thus involves a sleeve 21 inside of which there is arranged crystal 20. Two countercheeks, cooperating with sleeve 21, press two metallic electrodes 22 and 23 against the opposite flat faces 26 and 27 of crystal 20, which faces are perpendicular to the crystallographic axis of the crystal. Between the lateral surface of the crystal 20 and the interior surface of sleeve 21, there is a space 32 filled with an insulating liquid. Between thick metallic electrode 22 or 23 and the inside surface of sleeve 21, there is no sealing device and the insulating liquid in space 32 communicates with the insulating liquid filling the spaces between faces 26 and 27 on the crystal and protecting windows 50 and 51, respectively. The insulating liquid thus bathes almost all of the crystal 20 and is held by sealing joints such as joints 37 and 47, for example.

This liquid, is selected so as not to deteriorate the crystal 20 and the protecting windows 50 and 51, and its refractive index must be close to that of the elemenm 20, 50, 51, so as to prevent any loss of light due to parasite reflections. Finally, this liquid must be transparent to the wave length of the light beam traversing the cell.

These conditions interfere with obtaining high-quality optical cells and complicate the achievement of these cells because they considerably limit the choice of usable liquids.

All of the preceding devices involve protecting windows whose faces create parasite reflections and alter the transmission factor.

FIG. 3 is a cross-section view of a variation of the cell according to the invention. This cell involves a crystal 20, kept in a cylindrical body 21 by two metallic cheeks serving as electrodes 22 and 23 and two countercheeks 24 and 25 which can be made of an insulating material.

Mechanical systems 28 and 29 apply, without leeway, the electrodes 22 and 23 upon the crystal 20.

The voltage can be applied to the electrodes by means of two electrical connections 30 and 31, traversing countercheeks 24 and 25.

Faces 26 and 27 of crystal involve an anti-reflection coating for the wave length of the light beam traversing the cell resulting in the transmission factor of the cell being at a maximum. This coating also protects the faces of crystal 20 which are very hygroscopic.

According to the invention, the peripheral portions, supporting electrodes 22 and 23, and the lateral surface 32 of crystal 20, are bathed in an appropriate insulating liquid 33, such as, for example, a silicon oil.

This liquid only has an insulating function; it is therefore, not necessary to select its index as a function of that of the crystal.

Toric joints 34, 35, 36, 37 assure the tightness of the assembly while increasing the portion of the surface of the electrode 22 which is electrically insulated from the remaining elements.

One can readily apply to the electrodes of a cell, according to the invention, voltages exceeding 17K volts, while obtaining maximum optical transmission.

FIG. 4 represents another device according to the invention, enabling an electro-optical cell to be put together, which has a maximum transmission factor and an electrical insulation compatible with the very high voltages applied to the crystal.

A crystal 20 is held in a cylindrical insulating mounting 38, which can, for example, be made of a synthetic resin, by means of a sleeve adhering to the lateral surface 45 of the crystal and to the inside lateral surface 44 of mounting 38.

Sleeve 43 is made from a heat-hardenable synthetic substance or from a synthetic substance that is polymerizable at ordinary temperatures, such as, for example, a substance based on a liquid silicon elastomer which is polymerized at ambient temperatures. The sleeve 43 obtained here should form preferably a compact, flexible, elastic, and electrically insulating mass, molding the crystal without exerting on that crystal any mechanical stresses during or after the polymerization. The substance mentioned above by way of example is an excellent insulating agent and its elastic properties are such that crystal 20 will not show any strain after molding.

Two electrodes 39 and 40 are applied to the crystal 20 by means of two countercheeks 41 and 42. Conventional elastic recovery systems 46 and 47 cancel out the play between the crystal and the electrodes. Connections 48 and 49 conduct the feed voltage to the electrodes 39 and 40.

According to this invention, electro-optical cells involving several crystals, which are arranged optically in series in a common mounting, can be made without any difficulty.

FIGS. 5 and 6 represent such a variation corresponding to the case of FIGS. 3 and 4. In these figures, the intermediate electrode 22 or 39 involves a voltage lead-in conductor 52 or 53 and the voltage is applied between this intermediate electrode and the two other electrodes which are electrically connected to each other by a conductor, not shown.

This description was prepared with reference to a KDP crystal. It is obvious that the invention can apply to any electro-optical cell in which an axial electrical field must be created, regardless of the nature of the c stal.

Of course, the invention is not limit to the embodiments described herein and represented herein, which are given only by way of example. In particular, without going beyond the framework of the invention, detail modifications can be introduced; certain arrangements can be changed or certain means can be replaced by equivalent means.

What is claimed is:

1. An electro-optical cell comprising an insulating case having two openings arranged opposite to each other, crystal means having two flat faces perpendicular to its crystallographic axis and having a lateral surface, two insulating countercheeks supporting said crystal means within said case, said countercheeks cooperating with said insulating case and covering said openings, metallic electrodes positioned between said countercheeks and the faces of said crystal means, said electrodes and countercheeks having axially aligned openings to allow the passage of light through said crystal means, and a sleeve consisting of an electrically insulating liquid arranged around said lateral surface so that said sleeve is in contact with said lateral surface and the inner surface of said case opposite said lateral surface and so that axially aligned areas of said faces are free from said insulating liquid, whereby said light traverses said crystal means without passing through said liquid and said electrodes are electrically insulated from each other.

2. The electro-optical cell according to claim 1 further comprising sealing joints arranged between said electrodes and said case and between said electrodes and said crystal means and wherein the inside volume defined by said crystal means, case, electrodes and sealing joints is filled with said electrical insulating liquid.

3. The electro-optical cell according to claim 1 wherein said crystal means comprises a stack of individual crystals, each having two flat faces which are perpendicular to the crystallographic axis of said crystal and which are separated by intermediate electrodes, each of said crystals thus arranged in series having a sleeve made of said insulating liquid arranged around the lateral surface of said each crystal and in contact with this lateral surface and the inner surface of said case opposite said lateral surface so as to insulate electrically said electrodes from each other, axially aligned areas of said faces of said crystals being free of said insulating liquid whereby said light traverses said crystals without passing through said liquid. 

1. An electro-optical cell comprising an insulating case having two openings arranged opposite to each other, crystal means having two flat faces perpendicular to its crystallographic axis and having a lateral surface, two insulating countercheeks supporting said crystal means within said case, said countercheeks cooperating with said insulating case and covering said openings, metallic electrodes positioned between said countercheeks and the faces of said crystal means, said electrodes and countercheeks having axially aligned openings to allow the passage of light through said crystal means, and a sleeve consisting of an electrically insulating liquid arranged around said lateral surface so that said sleeve is in contact with said lateral surface and the inner surface of said case opposite said lateral surface and so that axially aligned areas of said faces are free from said insulating liquid, whereby said light traverses said crystal means without passing through said liquid and said electrodes are electrically insulated from each other.
 2. The electro-optical cell according to claim 1 further comprising sealing joints arranged between said electrodes and said case and between said electrodes and said crystal means and wherein the inside volume defined by said crystal means, case, electrodes and sealing joints is filled with said electrical insulating liquid.
 3. The electro-optical cell according to claim 1 wherein said crystal means comprises a stack of individual crystals, each having two flat faces which are perpendicular to the crystallographic axis of said crystal and which are separated by intermediate electrodes, each of said crystals thus arranged in series having a sleeve made of said insulating liquid arranged around the lateral surface of said each crystal and in contact with this lateral surface and the inner surface of said case opposite said lateral surface so as to insulate electrically said electrodes from each other, axially aligned areas of said faces of said crystals being free of said insulating liquid whereby said light traverses said crystals without passing through said liquid. 